The thing about this one, which sets it apart from on-pitch incidents, is that it was completely pre-meditated. Nobody can claim it was "in the heat of the moment", or just a misjudged move in a contact sport. No - he waited for him calmly on the tunnel afterwards, then headbutted him (a real headbutt not a Pardew-style head shove), and ran off like a coward.
I don't want to get over dramatic, but the way it was done off pitch and with planning, I'd say it becomes a matter for the police. That's common assault. It's no different to any member of the public waiting for a colleague outside of work and breaking his nose.